Organic Letters
Letter
between the allylation product and benzaldehyde proceeded
subsequently to afford (Z)-1-phenyl-N-(1-phenylbut-3-en-1-
yl)methanimine (11). In this reaction, allylic alcohol was
obtained only in a trace amount, thus verifying the affinity of
the allyl boronate with the imine over the aldehyde. The
reduction of 11 with NaBH3CN gave the known secondary
amine 12 in 63% isolated yield.
The feasible mechanism proposed was that the formation of
N−H imine and carbonyl compound proceeds by (i)
activation of the precatalyst, (ii) formation of a ruthenium(II)
azido alkoxide complex, and (iii) formation of a carbonyl
compound and an iminyl ruthenium complex via C−C bond
cleavage. The precatalyst (1b) has been studied and known for
photoactivation to afford a reactive ruthenium dimer (1bact)
via the release of CO (Scheme 3).10 Subsequently, the model
Scheme 3. Proposed Reaction Pathway for Ruthenium-
Catalyzed C−C Bond Cleavage of β-Hydroxy Azides
substrate (1-azido-1,2-diphenylpropan-2-ol, 2o) reacts to
cleave the reactive dimer generating ruthenium(II) alkoxide
complex (13) and a ruthenium(II) hydride (14) (Scheme 3).
The alkoxide complex (13) was assumed to afford a
ruthenium(II) azido alkoxide complex (15), which is an active
catalyst for the C−C bond cleavage reaction by photo-
irradiation. Once 15 was formed, the release of dinitrogen was
assumed to derive the formation of a highly reactive
ruthenium(II) alkoxy nitrene complex (16), followed by the
cleavage of C−C bond in the ruthenium β-hydroxy nitrene
complex via a transition state (TS1) to afford a carbonyl
compound and an iminyl ruthenium complex (17). The
substitution of the carbonyl compound and the iminyl ligand
with the substrate closes the catalytic cycle to generate the
ruthenium(II) azido alkoxide complex (15).
To evaluate the feasible mechanism for the transformation,
the proposed ruthenium complexes were computed by the
density functional theory (DFT) calculations using the
Gaussian 09 program11 package at the B3PW91/BSI//BSII,
calculation)12 in the gas and solution (THF) phase to obtain
an energy profile for the ruthenium catalysis. Figure 1
summarizes structural and free energy data for the proposed
pathway suggested in Scheme 3. For activation of the
precatalyst (1b) with the model substrate (2o), the formation
of two ruthenium intermediates (13 and 14) requires 19.2 and
21.4 kcal/mol in the gas and solution phase, respectively
(Figure 1b). The disproportionation of cyclopentadienylrute-
nium dicarbonyl dimer (1b) involves photodissociation of CO,
Figure 1. (a) Net thermodynamic energy for the formation of N−H
imine and carbonyl compound from β-hydroxy azide, (b) activation of
precatalyst, and (c) thermodynamic profile of a feasible mechanism
for ruthenium-catalyzed C−C bond cleavage of β-hydroxy azides with
schematic or optimized structures and relative free energies (kcal/
mol) computed at the B3PW91/BS I//BS II, SMD level in the gas
and solution (THF) phase.
subsequent ligand substitution, and cleavage of ruthenium
dimer by the substrates.10 After the ruthenium(II) alkoxide
complex (13) is generated, the substitution of CO with a
dangling azido group requires 11.2 and 15.9 kcal/mol in the
gas phase and THF solution, respectively (Figure 1c), to
produce the active catalyst (15). The catalyst (15) then release
N2 immediately to form a thermodynamically unstable
intermediate, the ruthenium(II) alkoxynitrene complex (16),
which produces a κO-carbonyl and iminyl ruthenium complex
(17) via a transition state of C−C bond cleavage with a barely
higher barrier (only 0.5 and 0.2 kcal/mol in gas phase and
THF solution, respectively). The κO-carbonyl and iminyl
ruthenium complex (17) converts to the active catalyst (15)
by deprotonation of the iminyl ligand subsequent ligand
substitution with the substrate (2o). Since the driving force for
this transformation involves the release of dinitrogen gas and
subsequent formation of the unstable nitrene ruthenium
complex, a net energy gain was calculated by 65 and 68
kcal/mol in the gas phase and THF solution, respectively.
D
Org. Lett. XXXX, XXX, XXX−XXX